Video-Bandwidth Electric field sensing using coherence modulation of light

Joel Santos-Aguilar, Celso Gutierrez-Martínez, Raúl Ochoa-Valiente, Misael Santiago-Bernal

Instituto Nacional de Astrofisica, Optica y Electrónica (INAOE) Apdo. Postal 51. 72000 Puebla, Pue. MEXICO

jaguilar@inaoep.mx, cgutz@inaoep.mx

Abstract—High intensity electric field detection can be realized by LiNbO3 crystals, in a simple configuration consisting of an electro-optic sensor and a low-coherence optical source. The electrooptic sensor introduces an optical delay which can be modulated (coherence modulation) by the wide-band electric field. A 0 - 5 MHz electric field sensing scheme using coherence modulation is reported in this work.

Keywords: LiNbO3, electro-optic sensors, electric field sensing

I. INTRODUCTION Electric field detection using electro-optic devices

represent alternative schemes to electric and electronic methods. The basic scheme of coherence modulation consists of a wide-band light source and an electro-optic retarder, an optical channel, and matched optical demodulator and a photo-detector, figure 1.

Figure 1. Electric field sensing based on optical coherence modulation

The optical source in this scheme is a low-coherence super luminescent diode. The electro-optic sensor introduces a static optical delay which depends of its physical length. The electric field sensor is an electrode-less LiNbO3 crystal. The sensed electric field modulates an optical delay which carries it through an optical fiber channel. At the receiver, the optical demodulator is matched to the optical delay and allows recovering the electric field, by measuring the autocorrelation of the received light. A previous work for coherence modulation in the 0 – 20 kHz range has been previously reported [1, 2].

In this work, we present a video-bandwidth electric field sensing scheme. The possibility to detect electric fields in the 0 -5 MHz range is attractive in practical instrumentation applications. The practical demonstration of the wide band detection is performed by sensing a composite video electric field.

II. THEORETICAL BACKGROUND The electro-optic sensor is a Z-cut, Y-propagating optical

waveguide which will introduce an optical delay and will

sense an external electric field. When the optical waveguide is used as a dielectric sensor, no electrodes are associated to the crystal and the electric field is present in the dielectric environment surrounding it (most commonly air ( 1rε = ), figure 2.

Figure 2. Dielectric electric field sensor

When a low coherence optical source is used, the electro-optic sensor introduce an optical delay or its equivalent optical path-difference ( 0dm ), greater than its coherence length [4]. In this work, the optical delays are generated by lithium niobate (LiNbO3) electro-optic devices acting as two-wave interferometers. Such devices generate static optical path-differences (OPD’s), which can be modulated by the sensed electric field. The optical retarders are Z-cut Y-propagating LiNbO3 birefringent optical guiding slabs, which introduce OPD’s as 45° polarized light is projected in orthogonal propagating modes TE and TM. Such modes travel in the slab at different velocities, as determined by the ordinary and extraordinary refractive index difference ( )eo nn − =0.083 at λ =1310 nm. A static OPD is then given as

( )0 m o ed n n L= − , L being the birefringent slab length.

On the sensing scheme, once the electric field is sensed, light is coherence modulated and transmitted through an optical fiber channel. At the receiver, light is demodulated using a second optical retarder, which measures the auto-correlation of the received light around the sensor’s OPD. This condition can be achieved by matching the OPD’s of the electro-optic sensor and the optical demodulator consisting of a PMF segment.

In the coherence modulated electric field sensing scheme, the electro-optic sensor introduces an OPD 0md , and the

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demodulator also introduces 0md . At the output of the optical demodulator, optical interference exists and the measured optical intensity corresponds to the normalized autocorrelation

of the transmitted light 0

2( ) ( ) cos( )g d g d dπλ

= , at the

optical path-difference 0md

At the output of the optical demodulator, which is matched to the electrooptic sensor, the optical intensisty is given as

0 00 0( ) ( )4 8

m mrI Id g dI = + (1)

Where 0( )mr dI is the received optical power. I 0 is the average emitted power of the optical source and 0λ is the center optical wavelength.

To ensure a linear detection of the imprinted electric field, the optical demodulator must be adjusted to a static optical path-

difference of 00 4( )md λ± . This adjustement is achieved either

by using a quarter-wave optical plate or by designing OPD= 00

4md λ± on the demodulator.

When an electric field )(tE z is sensed by the electrooptic sensor, it induces a dynamic variation Δd t( ) on the optical path-difference. On a z-cut LiNbO3 birefringent slab, the electric field is oriented on the Z-axis of the crystal, taking advantage of the 13r and 33r linear electro-optic coefficients. The time-varying optical path-difference is then given as

0( ) ( )md t d d t= + Δ (2)

Where 0 ( )( )2

zE td tEπ

λΔ = , Eπ is the half-wave electric field

given as

( )3 333 130 TM TEe o LE r rn nπ λ= −Γ Γ (3)

In expression (3), L is the electrooptic crystal length and represents the interaction length between the electric field and the optical wave, r13 and r33 are the electro-optic coefficients, ΓTE and ΓTM are the electric-optical overlapping coefficients. From expression (3), the half-wave electric field depends on the length of the electro-optics sensor. The longer the crystal, the lower the half-wave electric field and hence a higher sensitivity; e. g., the longer sensors will allow the measurement of lower field intensities. In practical applications, the longest crystals are of around 75 mm (3 inches), limited by the LiNbO3 commercial wafers sizes.

When the receiver is adjusted at the optical path-difference 0md , according to expression (1), the detected optical power

is given as

0 00 0( ) ( )( ) cos( ) ( )4 8 2 4 8

z zr

I E t I E tI It senIE Eπ π

ππ π= + − = +

Aditionally, if ( )Ez t Eπ<< , ( ) ( )( )z zE t E tsenE Eπ π

π π≈ the

measured optical power is given as

0 ( )( ) 14 2

zrI tEI t

Eπ

π= + (4)

Expression (4) represents an intensity modulation and the electric field is then detected as a linear variation of the received optical power. The half-wave electric field and the dynamic range depend on the length of the electrooptic sensor.

After eq. 3, the half-wave electric field, for LiNbO3 electrooptic sensors with L=10, 30 and 60 mm, at 0=1310 nm, are = 430, 144 and 72 KV/m, respectively. The optical transfer function in expression (4) for a 36 mm-long optical waveguide has been calculated and measured as depicted in figure 3. From this graph Eπ =120 KV/m.

Figure 3. Theoretical and measured optical transfer function of the

electrooptic sensor

In figure 3, the upper graph shows a dashed curve corresponding to a linear-scale measurement of the optical transfer function; the theoretical curve is also shown in continuous line. The lower graph in figure 3 shows the transfer function in logarithmic scale. The measured half-wave electric field is of about 112 KV/m, in good agreement to the theoretical value of 120 KV/m, as given by eq. (3). On the linear-scale, the optical transfer function shows a sinusoidal shape in agreement to eq. (2) and the regions of linear response can be identified. A first linear region is around 50 KV/m, a second one is at around 175 KV/m and so on. Linear sensing of electric fields can be considered, depending on the field intensity ranges and on the linear regions. From the

measurement of the optical transfer function, the dynamic range is of about 15 dB, as the measured electrooptic sensor is not fiber pigtailed. On a fiber pigtailed sensor, the dynamic range can be improved reaching up to 30 dB.

III. WIDE-BAND ELECTRIC FIELD SENSING Sensing video-band signals is an interesting application in

industrial facilities. In this work, for sensing video electric fields, a high voltage video amplifier providing dynamic electric fields in the 0-5 MHz range is used. The proposed experimental set-up is depicted in figure 4.

Figure 4. Wide-band electric field sensing scheme.

One condition for ensuring the detection of the electric field in the coherence modulation scheme is the matching of the optical path-differences between the sensor and the demodulator, in agreement to eq. 1. In figure 5 the autocorrelation measurement is shown. Figure 5a shows the optical path-differences of the sensor and demodulator.

Figure 5. Autocorrelation for a) sensor, b) demodulator .

Figure 6a shows the autocorrelation when the sensor and demodulator are in cascade. In this graph the delay matching can be seen. Figure 6(b) depicts a zoom on the matched fringe patterns.

Figure 6. a) Autocorrelation for the cascaded sensor and demodulator measured in b) a zoom of the fringe pattern around the matched path-differences.

On the matched fringes, the detection of the sensed electic field is ensured. If no fringes superposition exited, the sensed electric field could not be recuperated.

A dynamic 0-5 MHz signal is generated by a video high voltage amplifier, producing a video band electric field. In figure 7(a), a 5-MHz sinusoidal electric field has been sensed and measured. The upper curve corresponds to the input signal to the video amplifier, the lower curve corresponds to the detected electric field.

The measurement of a composite video signal electric field is shown in figure 7a. The upper signal in each figure corresponds to the input signal to the wide band amplifier. The lower curve corresponds to the sensed electric field at the output of an available video bandwidth photoreceiver. The electric field intensity is about 5 KV/m.

(a)

(b)

Figure 7. (a) 5 MHz sinusoidal electric field. (b) Video electric field

CONCLUSION

This work reports the electric field sensing of a 0 -5 MHz video-band electric fields, using coherence modulation of light. The sensing-detection scheme is based on matching optical delays which are generated by the electro-optic sensor and by the birefringent optical fiber demodulator. The sensing of a video signal has been successfully tested. These results open the perspective of sensing wide-band electric fields in the multi-MHz range, as found in high-voltage electric facilities and in industrial environments.

REFERENCES

[1] C. Gutiérrez-Martínez, and J. Santos Aguilar "Electric field sensing scheme based on matched LiNbO3 electro-optic retarders", IEEE Transactions on Instrumentation and Measurement, Vol. 57, no. 7, pp. 1362-1368, 2008.

[2] C. Gutierrez Martínez “Electric field sensing schemes using low-coherence light and LiNbO3 electrooptical retarders” - Christope Lethien: Editor, “Optical Fibre, New Developments”, 2009, In-The, ISBN 978-953-7619-50-3

[3] Fawwaz T. Ulaby, “Fundamentals of Applied Electromagnetics”, Chapter 4, Prentice Hall, 200.

[4] C. Gutiérrez-Martínez, J. Santos-Aguilar and R. Ochoa-Valiente, “An all-fibre and integrated optics electric field sensing using matched optical delays and coherence modulation of light”, Meas. Sci .Technol., 18(2007), pp. 3223-3229.